Negative pressure type booster

ABSTRACT

A negative pressure type booster, comprising a valve seat member ( 40 ), a spring ( 47 ) energizing the valve seat member ( 40 ), and a holding member ( 52 ) having an engaging part ( 52   b ) disposed on a power piston ( 22 ) movably only in radial direction and engaged with the valve seat member ( 40 ) and a tapered part ( 52   a ) in contact with a plunger ( 37 ), whereby, when the amount of movement of an input member ( 27 ) relative to the power piston ( 22 ) is larger than a specified value, the input member ( 27 ) is brought into contact with the tapered part ( 52   a ) and moves the holding member ( 52 ) in radial direction to disengage the holding member ( 52 ) from the valve seat member ( 40 ) in order to allow the valve seat member ( 40 ) to be moved rearwards.

TECHNICAL FIELD

This invention relates to a vacuum type brake booster for a vehicle.More particularly, the present invention relates to a vacuum type brakebooster for a vehicle that is capable of compensating for shortages inthe brake pedal depressing force during emergency braking operation.

BACKGROUND ART

There have been results of analysis that oftentimes, when a driverrapidly depresses the brake pedal because an obstruction has suddenlyappeared in the running path of the vehicle, the stepping force whichthe driver applies to the brake pedal is relatively small as comparedwith the stepping force required to produce a brake fluid pressure of amagnitude leading to a wheel lock, so that the braking capability of thevehicle is not fully exerted. On the basis of these results, it has beenproposed to install on the vehicle a device for determining, from thebrake pedal depression speed and the rate of increase of oil pressure ina master cylinder, whether the braking operation is a normal brakingoperation or an emergency braking operation, and when it is judged thatthe braking operation is the emergency braking operation, the brakefluid pressure is automatically raised up to a magnitude resulting inthe wheel lock.

Various constructions have been proposed for such the devices, and forexample one of these devices is described in Japanese Laid Open PatentPublication No. H11-48947 (1999). This discloses a vacuum type brakebooster that is constructed in such a manner that its input-outputcharacteristics can be switched by means of a control signal fromoutside between at least two different characteristics (a characteristicfor normal braking and a characteristic for emergency braking). With theinput-output characteristics of the vacuum type brake booster beingswitched from the characteristic for normal braking to thecharacteristic for emergency braking at times of emergency brakingoperation, it is possible to obtain the same result as when the driverdepresses the brake pedal strongly.

DISCLOSURE OF THE INVENTION

However, this conventional vacuum type brake booster includes a solenoidfor switching its characteristics from the normal braking operation tothe characteristics for emergency braking, and a valve seat memberdriven by the solenoid, apart from an annular seat valve for atmosphereinflow control. The solenoid driving signal is outputted from anelectronic control means for processing signals from a pedal strokesensor and a master cylinder oil pressure sensor. The solenoid is drivenon the basis of such the signals in a way that the valve seat memberhaving the annular valve seat for the atmosphere inflow control is awayfrom an opposed seal member and a valve mechanism is in an outputincrease state. This brake booster is considerably expensive because ofa necessity of the solenoid, several kinds of the sensors, theelectronic control means and the like.

Therefore, it is a primary object to provide a vacuum type brake boostercapable of obtaining an output increase state during the emergencybraking operation by a simple and cheap mechanical structure.

To achieve the object, there is provided a vacuum type brake boostercomprising: a housing for defining at least one pressure chambertherein; a movable wall member disposed in the housing to divide thepressure chamber into a constant pressure chamber and a variablepressure chamber and to be movable in the forward and backwarddirections with respect to the housing; a power piston connected to themovable wall member; an input member disposed in the power piston to bemovable in the forward and backward directions with respect to the powerpiston; an output member for outputting a propulsion force of the powerpiston in response to the movement of the movable wall outside thebooster; an annular valve seat for atmospheric air outflow controldisposed in the power piston to be directed rearwardly; a valvemechanism having a movable portion opposed to the annular valve seat foratmospheric air outflow control and a fixed portion secured air-tightlyto the power piston; the annular valve seat for atmospheric air outflowcontrol being secured to a valve seat member which is movable in theforward and backward directions with respect to the power piston;further, a holding means for holding the valve seat member at apredetermined front position in the case that an advanced distance ofthe input member with respect to the power piston is less than apredetermined value; a movable means for shifting the valve seat memberup to a predetermined distance at a rear position in the case that anadvanced distance of the input member with respect to the power pistonis more than the predetermined value; and a returning means forreturning the valve seat member at the predetermined front position inthe case that the power piston is in a predetermined position withrespect to the housing.

According to this booster, in the case that a relative movement of theinput member and the power piston is larger than the predeterminedvalue, i.e., in the cases of emergency braking operation wherein thedriver hurriedly depresses the brake pedal, an engagement of the valveseat member by the holding means is released and the valve seat memberis moved rearward up to the predetermined distance by use of the movablemeans, so that the valve seat member presses the movable member of thevalve mechanism rearward. As the result, the variable pressure chamberis rapidly and forcibly communicated with an atmosphere and a propulsionor thrust force more than the thrust force obtained during the normalbraking operation is applied to the movable wall member, the powerpiston and in its turn the output member to produce a sufficiently largemagnitude of the oil pressure for braking. In other words, by changing ajumping characteristic (when the braking operation is performed bydepressing the brake pedal, a ratio of the output force relative to theinput force becomes an infinity until the input member is abutted on thereaction member, this is referred to as the jumping), the thrust forcelarger than the thrust force as obtained during the normal brakingoperation is applied to the output member. Namely, the vacuum type brakebooster having two kinds of input-output characteristics for the normalbraking characteristic and the emergency braking characteristic isprovided by the simple structure.

Further, the wording of “rear” indicates the brake pedal side withrespect to the vacuum type brake booster or the vehicle's rear sideand,the wording of “front” indicates the master cylinder side or thevehicle's front side.

Preferably, the holding means is located at a portion of the frontinner-peripheral side of the power piston and has a holding member orholding members of which a front end is engaged with the power pistonand a rear end is engaged with the valve seat member. According to thisstructure, since the valve seat member is engaged with the holdingmember, which is engaged with the power piston, the valve seat member issecurely engaged with the power piston.

More preferably, one of the input member and the holding means isprovided with a tapered portion and the holding state of the holdingmeans is released by a cam action of the cam portion and the inputmember or the holding means, when an advanced distance of the inputmember with respect to the power piston is more than the predeterminedvalue. It is possible to rapidly shift the braking operation from thecharacteristic for normal braking to the characteristic for emergencybraking.

The annular valve seat for atmosphere outflow control is composed of afirst annular valve seat for atmosphere outflow control which isintegral with the power piston and a second annular valve seat foratmosphere outflow control which is aligned substantially with the firstannular valve seat and movable forward and rearward with respect to thepower piston. According to this structure, by separating the annularvalve seat for atmosphere outflow control from the first annular valveseat for atmosphere outflow control which is integral with the powerpiston and the second annular valve seat which is aligned substantiallywith the first annular valve seat for atmosphere outflow control, thevacuum type brake booster having two kinds of input-outputcharacteristics for the normal braking characteristic and the emergencybraking characteristic is constructed only by adding the second annularvalve seat for atmosphere outflow control to the power piston of theconventional brake booster having the annular valve seat for atmosphereoutflow control. This leads to a low price of the brake booster due tothe share of parts as used in the conventional brake booster.

The additional effects and objects of the present invention will becomemore apparent from the following detailed description with reference tothe accompanying drawings. Further, the term of “front” as used in thedescription indicates the front side of the vehicle and the term of“rear” as used in the description indicates the rear side of thevehicle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a vacuum type brake booster for avehicle constituting a first embodiment of the present invention;

FIG. 2 is an enlarged cross-sectional view of a main portion of thevacuum type brake booster shown in FIG. 1;

FIG. 3 is a cross-sectional view of a vacuum type brake boosteraccording to a second embodiment of the present invention;

FIG. 4 is a perspective view showing a valve seat member 40 beforeassembled;

FIG. 5 is a perspective view showing a holding member 52 beforeassembled;

FIG. 6 is a partially broken view of a vacuum type brake boosteraccording to a third embodiment of the present invention;

FIG. 7 is a perspective view showing a holding member 152 beforeassembled;

FIG. 8 is a cross-sectional view taken along the line X—X in FIG. 6;

FIG. 9 is a partially broken view of a vacuum type brake boosteraccording to a fourth embodiment of the present invention; and

FIG. 10 is a view showing a relation of a force (input) applied to aninput rod 28 and a thrust force (output) of an output member 55.

BEST MODE FOR CARRYING OUT THE INVENTION

The invention will now be described by way of examples with reference tothe accompanying drawings.

FIGS. 1 and 2 each is a cross-sectional view of a vacuum type brakebooster for a vehicle according to a first embodiment of the presentinvention. FIG. 2 is an enlarged view of a part of FIG. 1.

The vehicle vacuum type brake booster 10 (brake booster) in FIG. 1includes a housing 14 defined by a front side shell 11, a rear sideshell 12 and a movable wall 20 to thereby define within the housing 14 aconstant pressure chamber 23 which is communicated with a vacuum sourceand a variable pressure chamber 24 which is communicated with anatmosphere.

The movable wall 20 within the housing 14 is comprised of a metal plate18 and a rubber diaphragm 19 and disposed to be moved forward andrearward in the housing 14.

A bead at the outer edge of the diaphragm 19 is fixed in an airtightmanner by a folded portion of the outer periphery of the rear side shell12 and the front side shell 11. Also, a bead at the inner edge of thediaphragm 19 is air-tightly fixed with the plate 18 within a grooveformed on an outer circumferential periphery of the front side flange ofa power piston 22.

The constant pressure chamber 23 is connected to an engine intakemanifold (not shown), which serves as a negative pressure source, and isnormally held at a negative (suction) pressure during the engineoperation. The communication of the variable pressure chamber 24 and theconstant pressure chamber 23 is established or interrupted by a passage60 and a valve mechanism 36, and the communication of the variablepressure chamber with respect to the atmosphere is controlled throughthe valve mechanism 36.

As seen in FIGS. 1 and 2, defined within the power piston 22 are a firstspace 22 a positioned inside the front end (is FIGS. 1 and 2, left side)of the power piston 22 and connected with the constant pressure chamber,a second space 22 b positioned inside the rear end (in FIGS. 1 and 2,right side) of the power piston 22 and connected with the atmosphere,and a middle part 221 interposed between the first space 22 a and thesecond space 22 b. The middle part 221 has in its center a third space22 c connecting the first space 22 a to the second space 22 b, and apassage 60 connecting the first space 22 a to the second space 22 bseparately from the third space 22 c.

An input rod 28 and a plunger 37 constructing an input member 27 isdisposed inside the power piston 22 and passes through the first space22 a, the second space 22 b and the third space 22 c so as to be able toadvance/retract with respect to the power piston 22.

Also, the input rod 28 and the plunger 37 are integrally connected to aconnecting portion 80 and the input rod 28 is connected at its rear sidewith the brake pedal (not shown).

An output member 55 is fitted into the front side end of the powerpiston 22 and a reaction member 54 is disposed between the front sideend of the power piston 22 and the output member 55. Further, anabutment member 70 is disposed between the reaction member 54 and thefront side end of the plunger 37 and inside an engagement member 50positioned integral with the power piston 22 so as to be able toadvance/retract.

A key member 32 is provided for defining advance and retreat limitsposition of the plunger 37 and the input member 28 with respect to thepower piston 22. This key member 32 passes through a radial hole 33formed in a middle part 221 b the power piston 22. The front-reardirection (in FIGS. 1 and 2, left and right) thickness dimension of thekey member 32 is smaller than the front-rear direction dimension of theradial hole 33, and the key member 32 can move with respect to the powerpiston 22 in the front-rear direction through a distance C shown in FIG.2.

The key member 32 can abut upon the rear side shell 12 at the rear faceof both ends that projects radially outward by from the power piston 22,and the movement limit position of the power piston 22 with respect tothe housing 14 is a position in which the front side wall of the radialhole 33 abuts upon the front face of the key member 32 and the rear faceof the key member 32 abuts upon the rear side shell 12.

The middle part of the key member 32 is positioned between a pair offlanges 34, 35 formed on the middle part of the plunger 37, and themovement limit position of the plunger member 37 with respect to thepower piston 22 is a position in which the rear face of the flange 34abuts upon the front face of the key member 32 and the rear face of thekey member 32 abuts upon the rear wall of the radial hole 33. Theadvance limit position of the plunger member 37 with respect to thepower piston 22 is a position in which the front face of the flange 35abuts upon the rear face of the key member 32 and the front face of thekey member 32 abuts upon the front wall of the radial hole 33.

A valve mechanism 36 is disposed inside the power piston 22 andswitchable among an output decreasing action state, an output holdingaction state and a output increasing action state. In the outputdecreasing action state, the valve mechanism 36 connects the variablepressure chamber 24 to the constant pressure chamber 23 while cuttingoff the variable pressure chamber 24 from the atmosphere. In the outputholding action state, the valve mechanism cuts off the variable pressurechamber 24 from both the constant pressure chamber 23 and theatmosphere. In the output increasing action state, the valve mechanism36 cuts off the variable pressure chamber 24 from the constant pressurechamber 23 while connecting the variable pressure chamber 24 with theatmosphere.

This valve mechanism 36 is comprised of an annular valve seat 37 a foratmospheric air inflow control that is formed integrally with the rearend of the plunger member 37 and an annular valve seat 39 foratmospheric air outflow control that is formed integrally with the rearpart of the valve seat 40 around the rear opening of the third space 22c. The valve mechanism 36 also includes a cylindrical member 45 having amoving part 41 having valves 41 a, 41 b which face the annular valveseat 37 c for atmospheric air inflow control and the annular valve seat39 for atmospheric air outflow control, and a fixed part 43 that isair-tightly fixed to the power piston 22 by means of a retainer 42. Themoving part 41 is urged forward by a spring 44.

The valve seat member 40 is disposed inside the third space 22 c of themiddle part 221 and able to advance/retract with respect to the powerpiston 22.

As shown in FIG. 4, the valve seat member 40 has slit 40 b into whichthe abutment portion 37 c projecting radially from the plunger 37 isinserted, and a through hole 40 e extending in an axial direction of thevalve seat member 40. Also, the key member 32 is inserted into the slit40 b.

When the plunger 37 is inserted into the through hole 40 e of the valveseat member 40, the abutment portion 37 c and the slits 40 b arepositioned in opposed relation to each other so that the abutmentportion 37 c projects outside the valve seat member 40 through the slits40 b. When the plunger 37 with the valve seat member 40 is disposed inthe power piston 22, the outer surface of the abutment portion 37 cslidably abuts upon a concave portion 221 a of the inner circumferentialsurface of the middle part 221. The plunger member 37 is thus slidablysupported by way of the abutment portion 37 c in the front-reardirection on the inner circumferential part of the middle part 221, thatis, on the inner circumferential part of the power piston 22.

The valve seat member 40 is positioned between the plunger member 37 andthe middle part 221 of the power piston 22. At the rear end where theannular valve seat 39 for atmospheric air outflow control is formed, thevalve seat member 40 slidably abuts in the front-rear direction upon theinner circumferential surface of the middle part 221. That is, the powerpiston 22 has the middle part 221 as a holding part for slidably holdingthe valve seat member 40 in the rear direction.

An annular seal member 46 for maintaining an airtight seal between therear end part of the valve seat member 40 and the inner circumferentialsurface of the middle part 221 is fitted in the rear end part of thevalve seat member 40 which abuts upon the middle part 221. Also, thevalve seat member 40 is urged rearward by a spring 47 (urging member)which is disposed between the valve seat member 40 and the flange 34.

A spring 59 is disposed between a retainer 58 engaged with the input rod28 and a retainer 42 engaged with the power piston 22 to urge the inputrod 28 rearward. When the brake pedal is not depressed, the spring 59causes the annular valve seat 37 a for atmospheric air inflow control toabut on the valve 41 b of the movable portion or moving part 41 of thevalve mechanism 36 and hold the valve 41 a of the movable portion 41 ina state that the movable portion 41 a is away from the annular valveseat 39 for atmospheric air outflow control by a distance A.

The passage 60 and the first space 22 a formed in the middle part 221connect the valve mechanism 36 with the constant pressure chamber 23,and the third space 22 c connects the valve mechanism 36 with thevariable pressure chamber.

Inside the front part of the power piston 22, which is in the middlepart 221, a movable means is disposed for causing the valve seat member40 to retreat. This movable means is composed of the spring 47 (urgingmember) for urging the valve seat member 40 in the rear direction andthe key member 32.

A holding means 48 is disposed in the first space 22 a, namely aninterior of the front portion of the power piston 22. The holding means48 is composed of holding members 52 each having an engagement part 52engageable with an engaged part 40 c of the valve seat member 40 and aring like resilient member 53 for urging the holding members 52 radiallyinwardly to shorten its diameter. As shown in FIG. 5, the holding member52 is formed by two halves of a hollow truncated cone shape which isdivided into two pieces along a plain surface passing through its axisand two halves are assembled in the power piston 22 in the opposed form.An engagement part 52 c of each of the holding members 52 is fitted intoan engagement groove 50 a of the engagement member 50 secured to thefront portion of the power piston 22. Thus, it is impossible to move theholding members 52 in the forward and rearward directions, however, theholding members 52 can be radially moved around the engagement part 52c.

In FIG. 3, a second embodiment according to the present invention isillustrated. In the second embodiment, a first annular valve seat 38 foratmospheric air outflow control is integrally formed on an end facedefining a rear side opening of the third space 22 c of the middle part221 of the power piston 22 and a valve seat member 400 having a secondannular valve seat 390 which is opposed to the moving part 41 isdisposed to be movable in the forward and rearward directions withrespect to the power piston 22. The valve seat 400 is positioned insidethe first annular valve seat 38. Also, the structure other than thestructure as mentioned above is the same as that of the first embodimentas shown in FIG. 2.

FIG. 6 shows a third embodiment of the present invention wherein aholding means 148 is different from that of the first or secondembodiment. In the third embodiment, the holding means 148 is composedof holding members 152 each having an engagement part 152 b engaged withan engaged part 400 c of a valve seat member 400 and a ring shaperesilient member 53 for urging the holding member 152 in a direction tomake its diameter smaller.

A contact part 152 c of each holding member 152 is fitted into anengagement groove 150 a extending perpendicularly to an axis of thepower piston 22 while being formed on an engagement member 150 securedto the front end of the power piston 22. The holding members 152 can notbe moved in the forward and rearward directions, respectively, however,the holding members 152 can be moved radially around the engagementparts 152, respectively. As shown in FIG. 7, the holding members 152 areformed by dividing or cutting a hollow truncated cone shape member intothree pieces along its central axis. As shown in FIG. 8, the hollowtruncated cone that is divided into three pieces is assembled in opposedrelation with each other.

Further, a restriction member 170 of a cylindrical member having a thinthickness is disposed on the outer peripheral surfaces of the holdingmembers 152 so as to prevent the engagement parts 152 c from slippingout from the engagement grooves 150 a. Namely, even if a component ofaxial force is applied to the holding members 152 by a cam action due tothe abutment of the tapered surface 37 b of the plunger 37 and thetapered portion 152 a of each holding member 152, the engagement part152 c is not lifted out of the engagement grooves 150 a. Thus, theadvance distance of the plunger 37 with respect to the power piston 22is obtained without a loss so that a normal braking operation can beshifted speedily to an emergency braking operation.

As shown in FIG. 6, a plain surface, which passes through a central axisof rotation of each engagement part 152 c which is positioned in eachengagement groove 150 a and a central line X₃ of the abutment surface ofthe tapered part 152 a of each holding member 152 and the tapered part37 b of the input member or the plunger 37, is set in such a manner thatan angle of inclination with respect to the central axis X₃ of the powerpiston 22 is a 30-degree angle or less. This brings a distance B betweenthe tapered part 152 a of the holding member 152 and the central axis ofthe power piston 22 close to a distance A between the rotation center ofthe engagement part 152 c and the central axis of the power piston 22and prevents the holding members 152 from being rotated in the direction(clockwise in FIG. 6) to be engaged with a valve seat member 400 by anaxial component of force due to the cam action. This will be effectiveto achieve an object to promptly switch the braking character from thecharacter for normal braking to the character for emergency braking.

Said angle is not only applied to the third embodiment, but also to thefirst, second and fourth embodiments.

FIG. 9 shows the fourth embodiment of the present invention wherein aholding means 248 is different from that of the first, second and thirdembodiments. In the fourth embodiment, the holding means 248 is composedof holding members each 252 having an engagement part 252 b engageablewith an engaged part 400 c of a valve seat member 400, and the ringshape resilient member 53 for urging the holding member 252 to make itsdiameter smaller.

The engagement part 252 c of the holding member 252 is connected to themain body of the holding member 252 through a connecting part 252 b andformed in a cylindrical shape.

Also, an engagement groove 250 a is formed on an engagement member 250secured to a front portion of the power piston 22 and perpendicular tothe central axis of the power piston 22. The engagement groove 250 a isof a cylindrical inner hole into which the engagement part 252 c of theholding member 252 is inserted. Further, the engagement groove 250 a hasa cut-out portion 250 b extending along the engagement part 250 a.

The engagement part 252 c is inserted into the engagement groove 250 afrom the direction substantially perpendicular to the central axis ofthe power piston 022, then the connecting part 252 b connecting theengagement part 252 c and the main body of the holding member passesthrough the cut-out portion 250 b. Namely, the engagement part 252 c andthe engagement groove 250 a make a joint like connection. As the result,the holding member 252 is smoothly rotated around the central axis ofthe engagement groove 250 a or around the axis perpendicular to thecentral axis of the power piston 22. Because of the joint likeconnection, the engagement part 252 does not slip out from theengagement groove 250 a and an emergency braking operation of a highreliability will be executed.

Also, each holding member 252 has the same structure as that of thethird embodiment as shown in FIG. 7 excepts the engagement part 152 c.Namely, each holding member 252 is formed by dividing or cutting ahollow truncated cone member into three pieces or by cutting the hollowtruncated cone member into three plain surfaces passing through thecentral axis. These three pieces are assembled in the opposed relationwith each other as well as the third embodiment as shown in FIG. 8.

An operation of the vacuum type brake booster will be described,however, since an operation of the brake booster in the normal operationstate is well known, an explanation thereof will be omitted. Theoperation of the brake booster in the emergency braking state will bedescribed hereinafter.

The characteristic for emergency braking of the vacuum type brakingbooster according to the present invention is performed by changing ajumping character and applying a propulsion or thrust force to an outputmember, of which an amount is larger than a magnitude of a thrust forceunder the normal braking operation. In FIG. 2, to change the jumpingcharacter, a distance D between the abutment member 70 and the reactionmember 54 is preferably made larger. The enlargement of the distance Dis the same thing as the enlargement of the distance between theabutment surface 22 d of the power piston 22 with the reaction member 54and the annular valve seat 39 for atmospheric air inflow control, andthe enlargement of the distance between the abutment surface 22 d of thepower piston 22 with the reaction member 54 and the annular seat valve37 a for atmospheric air inflow control. Namely, the distance D is madelarger by moving the annular valve seat 39 and the annular valve seat 37a rearward, the abutment member 70 makes an output larger until areaction force from the reaction member 54 is received and a ratio ofthe output with respect to the input becomes infinite. The output underthe jumping condition becomes larger as compared with that in the normalstate.

The actual operation will be described on the basis of FIGS. 1 to 9.

Under the emergency condition where the driver depresses the brake pedalin a fluster, when an amount of a relative movement of the power piston22 with respect to the input member 27 is larger than the predetermineddistance B, the tapered surface 37 b of the plunger 37 is abutted on thetapered part 52 a of the holding member 52 and the holding member 52 isenlarged radially against the urging force of the ring shape resilientmember 53.

When a minimized internal diameter portion 52 aa of the tapered portion52 a runs on to a stepped portion 37 d of the plunger 37, the engagementof the engaged portion 40 c of the valve seat member 40 and theengagement part 52 b of the holding member 52 is released. Since thevalve seat member 40 is urged by the spring 47 (urging member) rearward,the valve seat member 40 is moved rearward by the spring 47 (urgingforce) soon after the engagement of to the engaged part 40 c isreleased.

When the valve seat member 40 is moved rearward, the annular valve seat39 for atmospheric air outflow control is abutted on a valve 41 a makingthe moving part 41 of the valve mechanism 36 and the communication ofthe constant pressure chamber 23 and the variable pressure chamber 24 isinterrupted. Since the plunger 37 is moving forward together with theinput rod 28 and the valve seat member 40 forces the moving part 41rearward, the annular seat valve 37 a for atmospheric air inflow controlis rapidly away from the valve 41 b making the moving part 41 b of thevalve mechanism 36 and the variable chamber 24 is communicated with theatmosphere. As the result, the communication of the variable chamber 24with the atmosphere and the interruption of the communication of theconstant pressure chamber 23 with the variable pressure chamber 24 arerapidly made. The distance between the abutment 22 d of the power piston22 which is abutted on the reaction member 54 and the annular valve seat38 for atmospheric air outflow control and also the distance between theabutment surface 22 d of the power piston 22 which is abutted on thereaction member 54 and the annular valve seat 37 a for atmospheric airinflow control are made larger. Thus, the output under the jumpingcondition becomes greater as compared with that of the normal state.

FIG. 10 shows the characteristics for normal braking operation and foremergency braking operation. In FIG. 10, the jumping in the normalbraking state is capable of obtaining the magnitude of the output F₀₁and the jumping in the emergency braking state is increased or raised upto the magnitude of F₀₁₁ so as to produce a large amount of the brakeoil pressure.

When the brake pedal is returned after the braking operation isterminated, the plunger 37 is moved rearward while the flange surface 34thereof is abutted on the key member 32. When the key member 32 isabutted on the rear side shell 12 (fixed member), the key member 32 isabutted on the front side groove wall 40 f (refer to FIG. 4) of thevalve seat member 40 so as to restrict the rearward movement of thevalve seat member 40 which is moving rearward together with the powerpiston 22. At this time, the rearward movement of the power piston 22 isfurther continued, the engagement portion of the holding member 52retreating together with the power piston 22 is engaged with the engagedpart 40 c of the valve member 40 which is abutted on the rear side shell12 (fixedmember) through the key member 32 and not moved. Thus, theengagement of the holding member 52 and the valve seat member 40 whichwas released by the emergency braking operation is again established andthe brake booster is ready for a next emergency braking operation.

INDUSTRIAL APPLICABILITY

According to the present invention, a low price of the vacuum type brakebooster which is simple in structure and has two kinds of input-outputcharacteristics for a normal braking and an emergency braking can beprovided without necessity of solenoids, all sort of sensors andelectronic devices.

What is claimed is:
 1. A vacuum type brake booster comprising: a housingfor defining at least one pressure chamber therein; a movable wallmember disposed in the housing to divide the pressure chamber into aconstant pressure chamber and a variable pressure chamber and to bemovable in forward and backward directions with respect to the housing;a power piston connected to the movable wall member; an input memberdisposed in the power piston to be movable in forward and backwarddirections with respect to the power piston; an output member foroutputting a propulsion force of the power piston in response to themovement of the movable wall member outside the booster; an annularvalve seat for atmosphere air outflow control disposed in the powerpiston to be directed rearwardly; a valve mechanism having a movableportion opposed to the annular valve seat for atmospheric air outflowcontrol and a fixed portion secured air-tightly to the power piston; theannular valve seat for atmospheric air outflow control being secured toa valve seat member which is movable in forward and backward directionswith respect to the power piston; further, a holding means for holdingthe valve seat member at a predetermined front position in the case thatan advanced distance of the input member with respect to the powerpiston is less than a predetermined value; a movable means for shiftingthe valve seat member up to a predetermined distance at a rear positionin the case that an advanced distance of the input member with respectto the power piston is more than the predetermined value; and areturning means for returning the valve seat member at the predeterminedfront position in the case that the power piston is in a predeterminedposition with respect to the housing; wherein the holding means isdisposed at the front end inner peripheral side of the power piston andhas an engagement portion engageable with the valve seat member.
 2. Avacuum type brake booster according to claim 1, wherein the holdingmeans includes a holding member which is engaged at its front end withthe power piston or a member on the power piston side and at its rearend with the valve seat member.
 3. A vacuum type brake booster accordingto claim 2, wherein the holding member is composed of a plurality ofmembers and a ring shape resilient member for urging the members inwardby a predetermined restraint force is provided.
 4. A vacuum type brakebooster according to claim 3, wherein three holding members areprovided.
 5. A vacuum type brake booster according to claim 1, whereinat least one of the input member and the holding means is provided witha tapered portion, the holding of the holding means is released by a camaction of the tapered portion and the holding means or the input memberwhen the advanced distance of the input member with respect to the powerpiston is more than the predetermined distance.
 6. A vacuum type brakebooster according to claim 5, wherein an angle of the inclination of aplane surface passing through a rotation center within the engagementgroove of the engagement part and an axis of an abutment surface of thetapered portion of the holding member and the tapered portion of theinput member is less than 30°.
 7. A vacuum type brake booster accordingto claim 1, wherein the annular valve seat for atmospheric air outflowcontrol includes a first annular valve seat for atmospheric air outflowcontrol integral with the power piston and a second annular valve seatfor atmospheric air outflow control movable in the forward and rearwarddirections with respect to the power piston and substantially coaxialwith the first annular valve seat.
 8. A vacuum type brake boosteraccording to claim 1, wherein the movable means includes at least anurging member for urging the valve seat member rearward.
 9. A vacuumtype brake booster according to claim 1, wherein the returning meansincludes a key member which is movable up to the predetermined distancein the forward and rearward directions and radially passes through thepower piston so as to restrict the rearward movement of the valve seatmember with respect to the power piston, and a fixed member secured tothe housing, the returning means causing the valve seat member to bereturned to the predetermined front position by the forward movementthereof with respect to the power piston.
 10. A vacuum type brakebooster according to claim 1, wherein the power piston is provided withan engagement groove extending perpendicularly to the axis of the powerpiston, and the holding member has an engagement part which engaged withthe engagement groove.
 11. A vacuum type brake booster according toclaim 10, wherein the engagement part of the holding member is formedsubstantially in a cylindrical shape, and the engagement groove of thepower piston is provided with a cut-out portion for allowing the holdingmember to rotate, the engagement groove is cylindrical inner hole intowhich the engagement part is fitted.
 12. A vacuum type brake boosteraccording to claim 1, wherein the engagement member which is one ofparts of the power piston is fixed to the front end of the power piston,and the holding means is disposed between the engagement member and thepower piston.
 13. A vacuum type brake booster according to claim 12,further comprising a restriction member for restricting an outwardmovement of the engagement part of the holding member.
 14. A vacuum typebrake booster for a vehicle comprising: a movable partition memberdefining a constant pressure chamber and a variable pressure chamber ina housing which is secured to a stationary member; a power pistonsecured to the movable partition member; an input member movable in theforward and rearward directions with respect to the power piston andreceiving an operation force from the outside; an output member foroutputting a thrust force of the power piston outside; a valve mechanismhaving a first valve member for introducing an atmospheric air pressureinto the variable pressure chamber and a second valve member forintroducing a section pressure in the constant pressure chamber into thevariable pressure chamber; a valve seat member movable into the forwardand rearward directions to actuate the valve mechanism; and a holdingmeans for moving the valve seat member rearward independent of themovement of the power piston when an advanced distance of the inputmember with respect to the power piston is move than the predetermineddistance; the holding means being able to have a factor or factors ofthe movement in a radial and/or axial direction, and the holding memberhaving an engagement part engageable with the valve seat member.
 15. Avacuum type brake booster according to claim 14, wherein the holdingmember is disposed between an inner peripheral surface of the hollowpower piston and the input member.
 16. A vacuum type brake boosteraccording to claim 15, wherein the valve seat member has a partengageable with the holding member, and an abutment of the part on theholding member causes at least a part of the holding member to beshifted in a radial and/or axial direction so as to release theengagement of the valve seat member and the holding member.
 17. A vacuumtype brake booster according to claim 14, wherein the abutment memberand the reaction member are disposed between the input member and theoutput member to make a space between the abutment member and thereaction member larger.
 18. A vacuum type brake booster according toclaim 17, wherein a spring for urging the valve seat member rearward atall times is disposed between the power piston and the valve seatmember.